When an image is dealt by a computer or the like, the image is expressed by pixels in a dot matrix state and each pixel is expressed by a gradation value. For example, a photograph or computer graphic is often displayed by pixels of 640 dots in the horizontal direction and 480 dots in the vertical direction on the screen of a computer.
On the other hand, the performance of a color printer has been being remarkably improved. The color printer has extremely high definition such as dot density of 720 dpi. In the case of printing an image of 640×480 dots in a corresponding manner on a dot unit basis, the image becomes very small. In this case, since the gradation values are different and the meaning itself of resolution differs, the image has to be converted to data for printing by interpolating the dots.
As conventional methods of interpolating dots in such a case, methods such as nearest neighboring interpolation (hereinbelow, called a nearest method) and cubic convolution interpolation (hereinbelow, called a cubic method) are known. Japanese Unexamined Patent Application No. 6-225140 discloses a technique of smoothing an edge in the event of interpolating dots by preparing a dot pattern by which an image is enlarged in a shape having a smooth edge.